Biofuels: The Drop-In Bridge

Every fuel so far requires a new ship or a shipyard. Biofuels require a bunker barge. They are the only decarbonisation lever that works on the fleet that already exists, in the tanks it already has, this voyage, which is why blends have become the fleet's first practical taste of the transition, and why every superintendent should understand them even if their ships never order anything more exotic.

FAME, HVO, and what "drop-in" really means

Two families dominate. FAME, fatty acid methyl ester, classic biodiesel, is made by esterifying vegetable oils, waste cooking oil or animal fats. It burns well in marine diesels but keeps oxygen in its molecule, and that oxygen brings housekeeping: FAME oxidises with age, absorbs water, can feed microbial growth in tanks, and thickens in the cold. None of this is disqualifying; all of it is management: tank turnover, water draining, monitoring. HVO, hydrotreated vegetable oil, takes similar feedstocks but hydrogenates them into a paraffinic hydrocarbon: a near-copy of diesel with excellent stability and no ester problems, commanding a price premium accordingly. Blends are described by percentage, B24, B30, over a conventional base fuel.

Chemistry aside, two regulatory ideas matter at the engine, and they are often run together. The first is a fuel-quality line: under the IMO's unified interpretation of MARPOL Annex VI (MEPC.1/Circ.795/Rev.9), a blend of up to 30% biofuel by volume is treated under regulation 18.3.1 like any petroleum fuel, while a blend above 30% falls under regulation 18.3.2, the paragraph for fuels not derived from petroleum refining, and the bunker delivery note must state how much biofuel the product contains. The second is the NOx question, and it has no percentage in it: a certified engine that burns a biofuel or blend, B100 included, without changing its NOx-critical components or settings outside its approved Technical File needs no NOx assessment at all. Only an engine that has to be altered to burn a blend above 30% must demonstrate NOx compliance, by onboard measurement or by reference to test-bed data. The 7% figure often quoted alongside these is not an Annex VI rule but the FAME allowance of the DF grades in ISO 8217:2017; the 2024 edition of the standard lifted that ceiling and added dedicated bio-residual grades. The unified interpretation (open; see its biofuel paragraphs) is short enough to read in full, and ABS's whitepaper on biofuels as marine fuel (open, 2021) covers the fuel chemistry.

The carbon is in the paperwork

Here is the conceptual heart of the lesson. A biofuel blend emits CO2 at the funnel like any hydrocarbon: tank-to-wake, nothing improves. The claim is well-to-wake: the carbon was recently absorbed from the atmosphere by the feedstock rather than lifted from geology. That means the emissions saving cannot be measured on the ship at all. It is established by certification of the supply chain, and the certificate is, quite literally, the product you are paying the premium for.

It also means different regulators can credit the same stem differently, and they do. Under the IMO's DCS and CII guidance, a biofuel earns a reflective emission factor when certified to deliver at least a 65% well-to-wake reduction against the fossil reference; fall short of the paperwork and it counts as fossil. Under EU MRV, feeding the EU ETS, the bio-share of a RED-certified fuel is zero-rated instead. Same molecules, two ledgers, two different numbers. DNV's regulatory note clarifying biofuel treatment across IMO DCS/CII and EU MRV/ETS (open) untangles this cleanly; the regulations themselves, CII, the EU ETS, FuelEU, are taught properly in Maritime Regulations, Explained.

A bridge with a width limit

Why "bridge" and not "destination"? Feedstock. Genuinely sustainable feedstocks, waste oils, fats, residues, are finite, and shipping bids for them against aviation and road transport, both of which can often pay more. Crop-based feedstocks raise land-use questions that sustainability criteria increasingly exclude. So biofuels scale to a meaningful share of marine energy, not to all of it; prices track feedstock scarcity, and fraud-resistant certification becomes as important as fuel quality. Lloyd's Register's Fuel for thought biofuel report (free after registration) covers the supply chain and quality landscape.

Operationally, the sensible entry path is unglamorous: start with a trial parcel, take representative samples at delivery, test the blend against the fuel standard and for compatibility with what is already in the tanks, and watch fuel-system behaviour, filters, separators, injection equipment, through the first weeks. Ships that treat the first biofuel stem as a small engineering trial rather than a press release accumulate exactly the operating record that makes the second stem routine.

On the framework: drop-in compatibility unmatched, availability decent and growing, energy density essentially conventional, safety unremarkable, and a climate value that lives entirely in certified paperwork over a supply chain with a hard ceiling. Use the bridge; just do not mistake it for the far shore.